Various designs and methods for prestressing structural members of the general type envisaged herein have been known and used for some time. See, for example, Canadian Pat. Nos. 581,580 issued Aug. 18, 1959 to Space Decks Limited; 843,058 issued June 2, 1970 to Luis R. Zamorano; and 950,630 issued July 9, 1974 to Edwin J. Cohen.
The 581,580 patent describes a space deck, for example, a roof or floor spanning a large distance. Such a space deck is said to include components each of which comprises a planar compression member, a junction unit spaced from the plane of the compression member and formed with a tension member securing formation positioned to secure a plurality of tension members in a manner restraining said junction unit from movement in any direction parallel to the compression member. A set of struts is connected between the junction unit and the compression member. The tension members may be threadedly connected to the junction units, to facilitate assembly and adjustment after assembly, for example, to introduce or remove a slight curvature in the assembled space deck. Such a feature is said to be important in large structures as it permits the complete elimination of deadload deflections.
Canadian Pat. No. 843,058 can be said to disclose a prestressed structural member. It discloses what generally could be considered as a latticed beam having diagonal members and a pair of spaced apart longitudinal members. The latticed beam of this patent is intended to be arcuate, i.e., curved. A tensile load is applied to one of the longitudinal members in a manner tending to flatten the curvature thereof, and in so doing, applies a pretensioning tensile force to the diagonal members and one of the longitudinal elements. It is important to note, however, that all elements of the latticework in this patent are "strictly in tension". This is readily apparent from the description on page 1 at lines 3-4, or on page 2 at lines 21-24.
Canadian Pat. No. 950,630 describes a method for erecting an arched, semi-flexible building member. Specifically, a longitudinal compressive force is applied to one longitudinal component of the latticed beam structure shown therein. That compressive force is effective to camber or bend the beam slightly upwardly to a predetermined extent within the elastic limits of that member. It is then locked in position with a predetermined amount of camber, i.e., curvature. In this particular patent, the application of a longitudinal tensile force to the longitudinal member closest to the center of curvature is said to cause the beam to be smoothly cambered upwards "without undesired buckling when the compressive force is applied". Thus, it appears that the diagonal members making up the latticework of the beam of this patent are prestressed in compression only.
These prior art techniques for prestressing the latticework of a structural member are therefore totally different from the prestressing method to be described herewith. These prior art techniques are thought to be limited to specific kinds of prestressed beams, namely, ones which are intended to resist only lateral loads.